The tests for GCM/GMAC and XPN for both encryption and decryption are very similar. The GCMVS generates 15 datasets for each supported combination of Key, Initialization Value (IV), Plaintext (PT), Additional Authenticated Data (AAD), and Authentication Tag length as described in GCMVS Configuration.
In the case of encryption, the Key, IV, Salt (XPN only), AAD, and PT are generated by the GCMVS, and the CT and Tag are generated by the IUT.
In the case of decryption, the CT and Tag are generated by the GCMVS, while the PT is generated by the IUT. In the decryption case the GCMVS also corrupts the IV, CT, or Tag ~50% of the time to ensure that the IUT can detect the errors. In the case of an error, the IUT is required to indicate a FAIL in its response. In the non-error case, the IUT must respond with the correct PT.